Oil filters are used in vehicles of various kinds (for example, in a transmission or a drive motor) to filter out foreign substances in the oil such as contaminants, particles, etc. The foreign substances become deposited as load on the filter medium, so that the filter function becomes adversely affected with increased loading. In order to properly maintain the filter function, it is conventional to replace the oil filter that is in use (or at least its filter medium, through which the liquid flows) at regular time intervals.